This invention relates to belts for use in the casting of metals and a method for the manufacture of such belts, and more particularly to belts suitable for use in the high speed continuous casting of aluminum alloys and methods for the manufacture of such belts.
The continuous casting of thin metal strips is generally known in the prior art, but has not been widely employed. Prior processes for the continuous casting of aluminum alloys into thin strip form have been limited to a relatively smaller number of alloys and products. It is generally recognized that, as the alloy content of various aluminum alloys is increased, the surface quality of the alloy as cast deteriorates.
Relatively pure aluminum such as foil can be continuously strip cast on a commercial basis principally because of the low alloy content. Similarly, building products have likewise been continuously strip cast; the surface quality of those products is less critical than in may other aluminum products such as can stock.
One conventional strip casting device which has been used in the prior art is the twin belt strip casting machine in which two moving belts define between them a moving mold for the metal to be cast. Cooling of the belts is typically effected by contacting a cooling fluid with the side of the belt opposite the side in contact with the molten metal. As a result, the belt is subjected to high thermal gradients, the molten metal being in contact with one side of the belt and the water coolant in contact with the other. Such gradients, dynamically unstable, cause distortion in the belts, resulting in neither the upper nor lower belt remaining flat. Those conditions adversely affect the surface quality of the metal cast.
As a result, belt casting techniques have not received wide spread acceptance in the casting of alloys for surface-critical applications such as the manufacture of aluminum can stock. Various improvements have been proposed in the prior art, including techniques in which the belts are preheated as described in U.S. Pat. Nos. 3,937,270 and 4,002,197, continuously applied and removed parting layers as described in U.S. Pat. No. 3,795,269.
It has also been proposed to perform continuous strip casting in single drum casters. In such devices, a supply of molten metal is delivered to the surface of a rotating drum, which is internally water cooled, and the molten metal is dragged onto the surface of the drum to form a thin strip of metal which solidifies on contact with the surface of the drum. Such drum casting also tends to have surface quality problems and various attempts have been made at solving those problems. For example, U.S. Pat. Nos. 4,793,400 and 4,954,974 suggest that the surface quality of the metal being cast can be improved by grooving the surface of the drums. A somewhat different approach was taken in U.S. Pat. No. 4,934,443 in which the deposition of the molten metal onto the surface of the drum, which may be grooved, establishes a natural oxide to develop on the surface of the drum as a result of exposure to the heat from the melt and to the atmosphere. Forming grooves in belts is, however, substantially more difficult than forming grooves on the surfaces of drum caster; because of inherent variations in belt steering and thickness, it is often difficult to control the spacing and depth of the grooves to be formed.
Substantial improvements in the strip casting of metals such as aluminum alloys are described in pending application Ser. No. 173,663 filed Dec. 23, 1990, as well as co-pending application Ser. No. 184,581 filed Jan. 21, 1994 and Ser. No. 173,369 filed Dec. 23, 1993, the disclosures of which are incorporated herein by reference. In the strip casters described in the applications, the apparatus includes a pair of endless belts, each of which is carried by a pair of pulleys. The belts define a molding zone therebetween corresponding to the desired thickness of the aluminum strip being cast. Aluminum alloy is supplied to the molding zone and solidifies therein. To prevent the substantial thermal gradients encountered in prior art twin belt casters, the apparatus described cools each of the endless belts while they are out of contact with either the molten metal or the cast metal strip. While the strip casting technique described in the aforementioned application represents a dramatic improvement over the prior art, it imposes severe constraints on the nature of the belt to be used. The belt used in that apparatus may run under conditions of high tension. The bending stress induced as the belts turn around their supporting pulleys combined with the tension stress on the belt require particularly high tensile strengths. It is not uncommon for such belts to grow in length by as much as 12 inches during 20 minutes of cast time. It was also found that, as described in U.S. Pat. No. 4,934,443, an oxide layer does not form on the belt until after the belt has been in use for some period of time. As a result, there is a tendency for the aluminum to adhere to the surface of the belt in initial start-up casting operations. Thus, the strip casting as described in the foregoing application imposes demanding requirements in terms of the properties of the belts used in the casting process.
It is accordingly an object of the present invention to provide belts for use in the casting of metals, and particularly aluminum alloy, and a method for the manufacture of such belts would overcome the foregoing disadvantages.
It is a more specific object of the invention to provide belts for use in the continuous casting of metals such as aluminum alloy in which the yield strength is dramatically improved to reduce stretching in the belt under elevated temperature conditions, while at the same time treating the surface of the belt to reduce the tendency for adhesion between the belt and the metals being cast.
It is yet another object of the invention to provide a belt for use in the casting of metals in which surface irregularities are introduced to the surface of the belt in contact with the molten metal to improve heat transfer therebetween and to allow the escape of gases to improve the surface characteristics of the metal being cast.
These and other objects and advantages of the invention appear more fully hereinafter from a detailed description of the invention.